Not so moist Mars: Clays may come from lava, not lakes

Hunting for Martians may be a tougher task than predicted. Clays, long thought to be a sure sign of a warmer and wetter past on the Red Planet, could merely signal earlier volcanic activity – which would have made some regions on Mars less favourable for life.

Clay layers found across Mars suggest that during the Noachian period, from about 4.2 billion to 3.5 billion years ago, the planet was warm enough to host liquid water – necessary for life as we know it.

Scientists thought Mars clays could have formed in one of two ways: through soil interacting with standing water on the surface, or from water bubbling up from below via hydrothermal vents.

"Both of those would have created habitats that would have supported microbes," says study co-author Bethany Ehlmann of the California Institute of Technology in Pasadena. "On Earth, microbes would have been thriving away, enjoying themselves."

Thick history

But a new analysis of Martian meteorites hints that some clays may not have formed the way we think.

Alain Meunier of the University of Poitiers in France has found that some Mars minerals from the Noachian period are a good chemical match to clays at the Mururoa Atoll in French Polynesia, which formed from cooling of water-rich lava.

What's more, these ancient Martian clays can be up to hundreds of metres thick, which is more likely to be associated with lava flows than soil interacting with water.

"Such a result would imply that early Mars may not have been as habitable as previously thought at the time when Earth's life was taking hold," wrote Brian Hynek of the University of Colorado in Boulder, who was not involved in the new work, in an accompanying commentary.

Tastes of lava

One way to confirm where Mars clays came from is to check the soil texture with a high-resolution microscope.

"Under each of those scenarios, there are particular characteristics of texture," Ehlmann says.

NASA's Curiosity rover has spent about a month in Gale Crater near the Martian equator, which holds a wealth of clay minerals. Curiosity has an onboard microscope, but it's not quite good enough to make the distinction.

Another option would be to do chemical analyses and look for certain rare-earth elements. But that would require a mission capable of returning pristine samples to Earth.

Still, Ehlmann is not worried about Curiosity's chances of finding clays made by liquid water. Gale Crater's morphology – the fact that "it was a big deep hole in the ground" – fits better with the theory that it was a lake, not a volcano.

"I think Gale is a different flavour of Mars," Ehlmann says. "If we wanted to test out this hypothesis, we'd head elsewhere."

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